Since the release of our Project 1 instructions on August 28th, our four-person engineering group has held several meetings to discuss our design and testing, while keeping an electronic design notebook until the project demonstration day on September 16th.
The project spanned around 1 month, and our final project turned out to be something really valuable, so I decided to record this experience here.
Design Goals:
The design goals for this project are to shoot accurately while being able to shoot as far as possible. Two tests are conducted during the project demonstration: the accuracy test and the maximum distance test.
During the accuracy test, the catapult needs to fire a projectile weighing 28.35 grams at a target 8ft away from the front-most part of the device while clearing a battlement with a height of 1.4 ft placed 1.5ft from the starting point.
During the maximum distance test, the catapult must launch the projectile as far as it can while still clearing the battlement placed at the same location as the first test.
The team's final score will be evaluated through a scoring equation:
Brainstorming and selecting the solution:
After our first meeting, we assigned the task of researching different catapult ideas to every group member, allowing all our ideas to contribute to the final idea, maximizing the advantage of group work.
The two most notable designs are shown above: a trebuchet with a winch and strings that pull down the catapult arm, and a regular catapult that launches by pulling a straw out.
Using an evidence-based decision matrix, we effectively selected the best solution that fits our needs. We weighed our customer needs that we identified for this project on a scale of 5 to 20, scaling from not-so-important (5) to very important (20). We then ranked each criterion of the two options on a scale from 1 to 10, with material usage, weight, and size being the smaller the higher the score, and structure simplicity, stability, and maximum distance being the greater the higher the score.
The total score of each option is evaluated by multiplying the weight by the scores and summing them up, resulting in our catapult being the better option.
However, we did modify some parts of the designs from the original brainstorming so that the best ideas are kept and combined into the best solution we think would be.
Solution Building Process:
Before constructing, we have 3D modeled the catapult in Onshape to get a hold of how the catapult would turn out to be like while confirming all the functionalities we have discussed are all included and feasible instead of having to build it out from scratch.
Since material usage is also an important part of this project, we wanted to be as careful as possible before starting the actual construction.
Catapult Solution:
Our final solution is shown above. The process of translating a 3D model into a real-life object caused us to encounter some problems, as force and structural stability were not as clearly demonstrated in the model because every component just sticks together like super glue.
The most prominent problem was the base not sticking together with straws because there wasn't enough surface contact, and the straw cuts were not clean enough to be fully attached to the popsicle sticks.
The other problem encountered was the winch not being able to hold the increasing tension of the catapult while winding the strings down when the winch is made using a straw. Straw was selected initially because its round characteristic allows a smooth rotation. Despite trying to fill it with glue, it did not provide enough strength, so we had to switch to stacking popsicle sticks to make an axis.
Our Final Demonstration
This was one of our demonstrations on September 16th. The catapult shot very accurately at the targeted point, but it was unfortunately one of the attempts for the max distance test and not the accuracy test.
Executive Summary
